add solution: merge-two-sorted-lists

Author: dusunax

merge-two-sorted-lists/dusunax.py

@@ -0,0 +1,77 @@
+'''
+# 21. Merge Two Sorted Lists
+
+A. iterative approach: use a temp node to store the result.
+B. recursive approach: use recursion to merge the two lists.
+
+
+## Time and Space Complexity
+
+### A. Iterative Approach
+
+```
+TC: O(n + m)
+SC: O(1)
+```
+
+#### TC is O(n + m):
+- iterating through the two lists just once for merge two sorted lists. = O(n + m)
+
+#### SC is O(1):
+- temp node is used to store the result. = O(1)
+
+### B. Recursive Approach
+
+```
+TC: O(n + m)
+SC: O(n + m)
+```
+
+#### TC is O(n + m):
+- iterating through the two lists just once for merge two sorted lists. = O(n + m)
+
+#### SC is O(n + m):
+- because of the recursive call stack. = O(n + m)
+'''
+class Solution:
+    '''
+    A. Iterative Approach
+    - use a temp node to store the result.
+    - use a current node to iterate through the two lists.
+    '''
+    def mergeTwoListsIterative(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        temp = ListNode(-1)
+        current = temp
+
+        while list1 is not None and list2 is not None:
+            if list1.val < list2.val:
+                current.next = list1
+                list1 = list1.next
+            else:
+                current.next = list2
+                list2 = list2.next
+            current = current.next
+        
+        if list1 is not None:
+            current.next = list1
+        elif list2 is not None:
+            current.next = list2
+
+        return temp.next
+
+    '''
+    B. Recursive Approach
+    - use recursion to merge the two lists.
+    '''
+    def mergeTwoListsRecursive(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        if list1 is None:
+            return list2
+        elif list2 is None:
+            return list1
+
+        if list1.val < list2.val:
+            list1.next = self.mergeTwoLists(list1.next, list2)
+            return list1
+        else:
+            list2.next = self.mergeTwoLists(list1, list2.next)
+            return list2